Genetic variation in resistance of Atlantic salmon (Salmo salar) to the salmon louse Lepeophtheirus salmonis

Susceptibility to the salmon louse Lepeophtheirus salmonis was recorded in three year-classes of Atlantic salmon with 300 (year-classes 2000 and 2001) and 50 (year-class 2001) full-sib families. The number of lice during natural infections was recorded on individual fish in all year-classes, while recordings from a challenge test were included for year-class 2002. Among the year-classes the mean number of motile lice per fish during the natural infection varied from 1.31 to 1.77, the number of sessile lice from 0.09 to 2.63 and the total number of lice from 1.40 to 4.40. During the challenge test the mean number of recorded lice was 74.6. The variation in the number of lice among individual fish was very high, with coefficient of variations larger than 60% for all recordings. The across year-classes heritability estimates for the number of motile lice recorded during the natural infections was 0.02 +/- 0.02, for the number of sessile lice 0.12 +/- 0.02 and for the total number of lice 0.14 +/- 0.02. The genetic correlations between the numbers of motile, sessile and total number of lice were all very high (r(g) greater than or equal to 0.98). The genetic correlations between body weight and these numbers of lice were of moderate size (0.32-0.37), showing that it is possible to improve both body weight and resistance to L. salmonis simultaneously through selection. The heritabilities for the number of lice recorded on naive fish were in general higher than when recorded on re-infected fish. The genetic correlations between these two lice counts were positive (0.26-0.35) but not significantly different from zero. The heritability for the number of lice recorded during a controlled challenge test was 0.26 +/- 0.07. The genetic correlation between the numbers of lice recorded in the challenge test and during a natural infection was very high (r(g)=0.88). It is concluded that the potential for improving resistance to sea lice in Atlantic salmon by selective breeding is good. As natural infection is highly variable in time and magnitude, it is recommended that challenge tests should be used in selective breeding to increase the resistance of salmon to L. salmonis.